The Curious Avalanches of Iapetus

Saturn's two-faced moon hosts extraordinary avalanches that cascade much longer than they should. Figuring out what makes them flow might help scientists better understand landslides on Earth.

Even among the fantastic variety that can be found in our Solar System, icy Iapetus is an odd duck. Saturn’s third-largest moon is half black, half white, a dichotomy that made it seem as though the moon vanished for half its orbit when discovered in 1671 by Giovanni Domenico Cassini. Iapetus also has an oddly squashed appearance, an equatorial bulging that could be explained if the moon revolved once every 16 hours — but Iapetus revolves once every 72 days. And on top of that, a mountainous ridge as high as 12 miles (more than twice the elevation of Mount Everest) circles much of the moon’s equator and makes it look like a giant walnut.

Now planetary scientists have another feature to add to the list of Iapetian oddities. While looking at high-resolution Cassini images, graduate student Kelsi Singer (Washington University in St. Louis) found evidence of 30 avalanches on Iapetus. Oddly, these chaotic cascades are much longer than they should be, a characteristic that may help scientists understand similar landslides on Earth.

As rock tumbles down a slope in an ordinary landslide, gravitational potential energy turns into ferocious motion that bulldozes just about anything in its way. Friction typically slows the rock to a stop before it can cover horizontal ground more than twice the height that it fell. But in sturzstroms (German for “falling stream”), a landslide becomes nearly unstoppable, flowing rather than sliding or tumbling, sometimes along nearly horizontal surfaces or even uphill. Sturzstroms can travel as much as 10 times their drop height. On Iapetus’s rugged terrain, that can result in landslides that surge as far as 80 km (50 miles).

An ice-moon might be an unlikely place to look when trying to understand sturzstroms on Earth. After all, we’re familiar with the slippery ice that has been the downfall of many a skater in the rink, so we would expect rocky landslides to be very different from icy avalanches on Iapetus. But Iapetus lives in a different part of the solar system, one on which it’s typically –280° Fahrenheit. At such low temperatures, ice is no more slippery than beach sand, which makes Iapetus a good laboratory for understanding sturzstroms on Earth.

Sturzstroms must stream along with reduced friction, but how? Geologists have at least nine different working theories — the leading one proposes that the avalanching material generates sound waves, changes in pressure that help the rock slip downward.

If sound waves are to blame, then geologists expect to see a trend — the bigger the avalanche, the more slippery the falling material. But on Iapetus, even the biggest avalanches are no more slippery than their smaller cousins.

“[The sound wave theory] is a fascinating possibility for landslides, and it is not ruled out completely by our data, but it is not required to explain our data either,” says Singer.

Instead, as detailed online in July 29th’s Nature Geoscience, Singer and her colleagues prefer a simple explanation called “flash heating.” As the falling surface rubs past the mountainside, the friction of the fall heats the ice enough to make it slippery — but not enough to melt it. The scientists suggest that even mildly heating the ice would transform the frigid, beach-sand ice into the slippery stuff we’re more familiar with. A similar flash heating mechanism might explain Earthbound sturzstroms, a claim supported by the discovery of melted rock found near landslide sites on Earth, imaginatively named “frictionite.”

Singer emphasizes that more observations are necessary. Iapetus’s low gravity might affect the results, so observations of avalanches on other icy bodies would help firm up the conclusions. Experiments in the lab could also test whether rapid motion makes super-cold ice more slippery.

“One of the exciting things about getting new data for the outer solar system is that we find interesting things we don’t expect,” says Singer. “We didn’t expect to find these fantastic landslides on Iapetus, but we were very excited that we did!”